After wells have been completed to the depth of one or more subsurface production zones and the zones have been determined to contain producible quantities of petroleum products, completion of the wells is often accomplished by gravel packing or propping activities wherein a fluid containing a quantity of sand, gravel and other proppant materials is injected into the well at high pressure and high rates of injection with injection being accomplished in the downhole environment in the immediate region of the production formation. When fluidized proppant materials are injected into the well under high pressure, the subsurface formation can develop fractures that extend radially outwardly from the well bore. When these fractures occur proppant materials such as sand and gravel will be caused to flow into the voids developed by the fractures and will fill the void and provide a porous support for opposed surfaces of the fractures as well as defining efficient flow paths for conducting petroleum products to the well for production. The porous support of the proppant material will permit petroleum products to flow from the formation into the fracture and through the proppant materials to the well bore for production through production tubing that will be installed as the final step of the completion activities.
Bottomhole pressure measurement is a valuable asset when performing formation propping activities, also known as enhanced prepacked completions. The types of pre-treatment tests recommended for enhanced prepacked completions are defined as are the types of well-site specific analysis values that are derived from each test. Additionally, various bottomhole pressure measurement techniques have been used in the past but these techniques typically have the short coming of being unable to provide pressure measurement and other data acquisition at a well depth below the crossover ports of the injection apparatus. For the reason that the pressure below the crossover ports, during proppant injection, is often in the range of 200 to 300 psi less than the pressure at the crossover ports, pressure measurement at or above the crossover ports can have considerable error.
Most well completion and production organizations advocate pre-treatment tests prior to performing an enhanced prepack well completion. The pre-treatment tests determine well-site specific values to insure the most effective completion will be provided. Analysis of pre-treatment tests, and the subsequent completion, is based on bottomhole pressure (BHP) data. BHP data improve analysis accuracy by eliminating surface pressure data interpretation errors due to fluid frictional effects and changing hydrostatic head as slurry concentration changes during the job. Industry options for BHP measurement include: Direct; real-time and memory downhole quartz crystal gauge, and Indirect; static annulus and computer modeling. There are prerequisite considerations for each of these techniques.
Industry clearly acknowledges that the most accurate method of BHP measurement at the present time is accomplished through the use of downhole quartz crystal gauges. Gauge location within the completion string has recently been gaining attention because pump rates have increased from a range of about 2 to 5 barrels per minute (bpm) to pump rates of 10 bpm, and higher. Gauge locations include; above, or below the crossover tool (fixed and non-retrievable) and placement within the washpipe. There are BHP data accuracy advantages to locating the gauge bundle carrier below the crossover port. In cases where the pressure gauge bundle is intended to be wireline retrievable however there has heretofore been no mechanism available for selectively locating the gauge bundle carrier below the crossover port and then providing for its subsequent retrieval independently of the gravel packer tool.
Prior to performing any enhanced prepack completion a suite of pre-treatment tests are recommended. Generally, these tests are performed immediately after perforating the well casing by locating downhole gauges in the well in the region of the casing perforations. It is accepted that BHP measurement with downhole quartz crystal gauges will provide more accurate analysis values. Real-time access to gauge measurements (electric line) is beneficial for applications where step-rate analysis indicates fluid leak-off will prevent formation fracture at maximum equipment pump rates.
During enhanced prepacking pre-treatment tests, and the subsequent completion, real-time bottomhole pressure can be determined directly from a quartz crystal gauge, or directly from a static annulus, or computer modeling. However, there are limitations for each of these techniques. During proppant stages, direct measurement techniques can employ gauges mounted to the exterior of the completion string to prevent the sensor and electric line from being damaged. If maximum casing pressures are of concern, the indirect static annulus method may not be applicable. If the formation will not support the hydrostatic head, neither the indirect static annulus or the computer model can be realisticly used.
Another method for obtaining recorded bottomhole pressure is via downhole memory gauges. When memory gauges are utilized, life requirements must considered as they are battery operated and have a maximum memory size. Memory gauges can be preprogrammed with a start data collection time and frequency of data collection. A minimum of two memory gauges are recommended for shallow depth wells and three gauges for deeper wells. It is recommended to stagger start times and frequency times. Five second sampling time is considered maximum for dynamic well conditions.
Memory gauges are termed non-real time as recorded data its accessible only when the gauges are retrieved at the surface. Generally, bottomhole data from memory gauges are not available for analysis until post-job. If real-time bottomhole gauges are not available, or static string measurement is not available, memory gauges are required as a minimum to support alternative bottomhole pressure recording measurements. The same mounting considerations apply here as with real-time gauges previously discussed.
In the past, a real-time quartz crystal gauge assembly has been mounted in a gauge carrier above the packer/crossover tool assembly. When a gauge carrier is used in this manner, the gauge or gauges are physically mounted externally of the pump-in tubing string and above the depth of the packer and thus can only sense bottomhole pressure in the annulus between the tubing string and casing at a depth above the packer. In this case, to provide real-time BHP data capabilities, an electric line is run to transmit data to the surface read out equipment from the pressure gauges. A pressure data acquisition system of this nature is incapable of measuring bottomhole pressure at a depth below the depth of the packer/crossover tool assembly.
In other cases a memory quartz crystal gauge assembly can be mounted in a gauge carrier and assembled to the tubing string above the packer. This pressure gauge assembly will be battery powered and will acquire downhole pressure data in accordance with a timed data acquisition sequence. Obviously, since the pressure gauge is secured to the tubing above the packer it is only capable of pressure detection in the annulus above the depth of the packer. Bottomhole data acquisition has not been previously available at a depth below the packer/crossover assembly through use of pressure gauge equipment of this nature. Additionally, the data from the pressure gauge assembly can be acquired for analysis only after the injection string has been recovered from the hole.
Another method that has been used for acquisition of BHP data is to provide for measurement with a static fluid analysis. In this case a gravel pack packer/crossover assembly is provided having a pressure gauge mounted for detection of bottomhole pressure at the bypass courts of the crossover tool. Bottomhole pressure measurement with this type of equipment can only accurately record bottomhole pressure when pumping activity is static. When high fluid rate pumping activity is in progress there will exist a significant pressure drop across the bypass ports so that pressure measurement during pumping activity will often exhibit significant error. Due to industry demands of increased completion pump rates, 10 bpm and higher gauge location within the completion string is considered of significant importance. At higher pump rates, dramatic turbulence is generated at the crossover port where the fluid being pumped down the string exits to the annulus between the tubing and casing below the depth of the packer and enters the perforated zone. The turbulent fluid activity generates additional frictional effects. If the gauge bundle carrier is located in the completion string, above the packer, the actual BHP of the perforated zone may be disguised. This disguise may result in inaccurate BHP analysis, specifically the tip screen-out associated with enhanced prepacking. Locating the downhole gauges below the crossover port is more critical with high leak-off carrier fluids. High fluid leak-off rates are generally associated with. HRWP, in which more than one tip screen-out may occur.
Although gauge location will affect the pre-treatment test data analysis previously discussed, it becomes more prominent when gravel laden fluid, or slurry, is pumped. Based on data presented to the industry, it is becoming increasing clear that for improved BHP data monitoring, and subsequent analysis gauges should be located below the crossover port when performing one of the enhanced prepacking techniques. This assumes no changes occur to the crossover port design to minimize frictional effects.
Placement of downhole gauges below the crossover port may consider two different arrangements. The first arrangement may consider the bundle carrier positioned between the crossover port and the top of the screen. The second arrangement may consider the gauge bundle carrier positioned in the washpipe. In most cases, positioning the bundle carrier in the washpipe places the gauges within the perforated interval. However, limitations may exist for washpipe positioning, such as washpipe diameter. Here again, in the event it is desired for the gauges to be retrievable without necessitating retrieval of the tubing string, no known procedure has been previously available for accomplishing both location of data gauges below the depth of the packer and crossover tool and also provide for retrievability of the gauge bundle carrier independently of the tubing string. It is considered quite desirable to provide a downhole well data acquisition system having the advantages of retrievability and also having the advantages of locating data gathering instruments such as pressure gauges within the tubing and at a desired depth below the packer and crossover assembly.